This invention relates to a process for preparing vinyl chloride polymers by discontinuous polymerization in the presence of oil soluble or water soluble catalysts and emulsifiers, as well as polymerization adjuvants.
The polymers obtained according to the process of this invention are particularly suitable for the preparation of plastisols with very low vicosities, not only in the low but also in the high shear range. This effect is especially desirable in the processing of plastisols by coating processes. Low paste viscosities in the lower shear ranges facilitate the handling of the pastes, e.g., transporation and screenability, and are self-venting during processing. Low paste viscosities in the high shear range permit high processing rates. Plastisols prepared from the polymers produced according to this invention have high stability in storage.
It is known to prepare polyvinyl chloride suitable for the production of pastes by continuous or discontinuous processes.
The continuous process produces a PVC paste with a desirably low viscosity in the high shear range, but with a high viscosity at low shear velocities. Furthermore, such polyvinyl chlorides have a high proportion of emulsifiers. High emulsifier concentrations are detrimental in the end products and in processing. They reduce the transparency and increase the sensitivity to water of the polymers and the articles produced from them.
In the discontinuous process, in principle two process variants are knwon. By the so-called microsuspension process, latices with a relatively low solids content of about 40% by weight are obtained. The spray drying of these latices therefore results in increased drying costs. The spray dried products result in pastes with a pronounced dilatant flow behavior. They are thus poorly suited for the application of spread-coating methods.
A two-stage microsuspension process is known from FR 20 86 635, describing the production of PVC latices with high solids concentrations. The operation of the process, however, requires the preparation of a seed latex in addition to the step of prehomogenizing and is thus expensive.
The discontinuous operation of the emulsion polymerization of vinyl chloride yields polymers with low emulsifier contents. In polymerizations with low amounts of emulsifiers to obtain stable latices and paste-forming polyvinyl chlorides with low paste vicosities, it has been proposed to use the so-called seed latex method. The method consists of providing a seed latex (germ latex or primary dispersion) prior to the emulsion polymerization. This process is, however, highly expensive on a large industrial scale, as the primary dispersion must be prepared separately, stored and dosed. Furthermore, the stability that may be obtained with the latices prepared in this manner is not optimal and the viscosities of the polyvinyl chloride pastes obtained by this method are clearly higher than the viscosities of pastes obtained by the continuous process.
According to the process of DE No. P 1964 029 and the laid-open text of Belgian patent No. 656 985, the emulsifier is added during the polymerization process according to a definite program. It is possible by this procedure to prepare polyvinyl chloride with particularly low emulsifier contents. But the polyvinyl chloride pastes obtained by this process have an even significantly higher viscosity than those obtained by the continuous process.
Discontinuous processes are known which use dispersing agents (fatty alcohols). The following can be cited to illustrate the state of this art:
1. DE-OS No. 22 60 957 (BP No. 1383100)
2. DE-AS No. 26 29 655 (U.S. Pat. No. 4,310,649)
3. DE-OS No. 28 50 105
4. DE-OS No. 27 42 178 (U.S. Pat. No. 4,093,581)
5. EP No. 00 30 524
6. DE Application No. P 32 10 891.5 (U.S. Ser. No. 478,766 of Mar. 25, 1983)
The disclosures of all of these references are incorporated by reference herein.
The polymerizations of processes 1, 2 and 3 are effected using oil soluble activators, in part with the addition of reducing agents. These processes lead to products which, when processed into plastisols, exhibit strongly dilatant flow characteristics, as shown by the comparative experiment A of German application No. P 32 10 891.5 and its U.S. equivalent.
Process 4 uses inorganic catalysts for activation. According to 4, a mixture consisting of a C.sub.16 -C.sub.20 alkyl alcohol and an alkyl sulfate (C.sub.12 -C.sub.18) is used in the polymerization. As shown by comparative experiments B and C of the German application No. P 3210891.5 and its U.S. equivalent, only unstable latices or latices low in solids are obtained by process 4. After spray drying, these yield a polyvinyl chloride resulting in high viscosity pastes with a pronounced pseudoplastic flow behavior.
According to process 5, a water soluble substance is initially homogenized in the presence of water and an emulsifier. Subsequently, the monomer and the initiator are added. Successful execution of this proces requires the presence of a seed latex (stabilizing latex) and the process is thus relatively expensive.
According to process 6, an emulsifier system consisting of a surface active agent and a long chain alcohol is added to the polymerization formulation which uses inorganic initiators. As shown by comparative experiment B herein, a polyvinyl chloride yielding low viscosity pastes is obtained but the storage stability of the pastes is not optimal.